All orbital elements need to be defined in a coordinate system and at an epoch (specified time).
Earth Centered Inertial (ECI) -
Earth Centered, Earth Fixed (ECEF) -
A reference time, or timestamp, in which time-varying celestial data is defined.
A commonly used epoch is J2000 - January 1, 2000 at 11:58:56 GMT
ECI vs ECEF – Orbital Tracks:
The ECI and ECEF coordinate frames both have their advantages and disadvantages. When defining the orbital parameters of a satellite, the ECI coordinate frame is largely superior. The video to the right demonstrates why.